One or more embodiments of the present invention pertain to method and apparatus for performing an optical coherence tomographic examination of tissue such as an eye. In particular, the present invention relates to method and apparatus for performing an optical coherence tomographic examination of an eye using an active tracking system to lock an optical coherence tomography (xe2x80x9cOCTxe2x80x9d) scanning beam on desired features in retinal tissue for use, for example, in imaging retinal tissue, measuring retinal and retinal nerve fiber layer thickness, mapping the topography of the optic nerve head, and so forth.
As is well known, an optical coherence tomography (xe2x80x9cOCTxe2x80x9d) apparatus (for example, as disclosed in U.S. Pat. No. 5,321,501 (xe2x80x9cthe ""501 patentxe2x80x9d) is an optical imaging apparatus that can perform micron-resolution, cross-sectional imaging (also referred to as tomographic imaging) of biological tissue. As is also well known, to make measurements along an axial direction (i.e., into the biological tissue): (a) radiation is directed to, and reflected by, a reference mirror located in one arm (a reference arm) of a Michelson interferometer (the position of the reference mirror is scanned); and (b) in a second arm (a sample arm) of the Michelson interferometer, radiation is directed to, and scattered by, the biological tissue. Whenever the optical path difference of radiation in the two arms of the Michelson interferometer equals, or is less than, the optical coherence length of the radiation transmitted into the interferometer from a source, an optical interference signal can be detected. As disclosed in the ""501 patent, a cross-sectional image of the tissue is formed by combining data from serial axial scans.
The length of time it takes to produce a tomographic image is limited by several factors: (a) the scan speed of the reference mirror in the reference arm used to obtain measurements in the axial direction; (b) the transverse scan speed of deflectors used to acquire serial axial scans; (c) signal-to-noise limits related to image quality; and (d) the speed of electronics, and any associated computer, in sampling analog OCT signals and transforming them into a pseudo color, or gray scale, image. However, in general, as the scan speed of the reference mirror goes up, the signal-to-noise ratio goes down; thereby adversely affecting the image quality. On the other hand, when imaging tissue in an eye, one is constrained to obtain images rapidly to avoid problems caused by eye movement.
At present, the scan speed of the reference mirror is a limiting factor in OCT image acquisition. To understand this, refer to U.S. Pat. No. 5,459,570 (xe2x80x9cthe ""570 patentxe2x80x9d) where the reference mirror is moved by a PZT actuator. Although the scan speed of a PZT actuator can be as high as several KHz, the scan range is limited to the micron range, which micron range is not practical for in vivo human eye diagnosis where a scan range of a couple of millimeters is required for clinical use. Although the required several millimeter scan range can be obtained by mounting a retro-reflector on one end of an arm that is scanned by a galvanometer, the scan speed is limited to about a couple hundred hertz (this scan method is currently employed in a commercially available OCT scanner device made by Zeiss Humphrey Systems of Dublin Calif.).
A scan device in an OCT system that provides a two to four KHz scan speed with a useful scan range was disclosed in an article entitled xe2x80x9cHigh-speed phase-and group-delay scanning with a grating-based phase control delay linexe2x80x9d by G. J. Tearney et al. in Optics Letters, Vol. 22, No. 23, Dec. 1, 1997, pp. 1811-1813, which scan device was based on a phase ramping delay line principle disclosed in an article entitled xe2x80x9c400-Hz mechanical scanning optical delay linexe2x80x9d by K. F. Kwong et al. in Optics Letters, Vol. 18, No. 7, Apr. 1, 1993, pp. 558-560. A disadvantage of the scan device disclosed in the G. J. Tearney et al. article is that it is easily worn out, and there is an upper limit light power allowed for safe use in in-vivo human eye diagnosis. However, as pointed out above, with increasing scan speed, the signal-to-noise ratio will be reduced, and image quality will deteriorate.
Although OCT scan data can be used to provide tomographic images of tissue such as an eye, the OCT data obtained has many uses other than in providing an image. For example, applications of OCT data include measuring retinal and retinal nerve fiber layer thickness, mapping the topography of the optic nerve head, and so forth. However, in these applications, similar problems arise, i.e., how to obtain data having acceptable signal-to-noise ratios while taking into account movement of the tissue. In light of the above, there is a need for a method and apparatus that can obtain high quality OCT data, for example, to form tomographic scan images, while taking into account the issue of, for example, patient movement.
One or more embodiments of the present invention advantageously satisfy one or more of the above-identified needs in the art, and provide method and apparatus for performing optical coherence tomography (xe2x80x9cOCTxe2x80x9d) applications. Specifically, one embodiment of the present invention is an OCT application apparatus that performs an OCT application on an object, which OCT application apparatus comprises: (a) an OCT scanning apparatus which outputs a scanning beam of OCT scanning radiation; and (b) an active tracking system that generates and projects a tracking beam of tracking radiation onto a region including a reference tracking feature; wherein the active tracking system further comprises an analysis system that analyzes tracking radiation reflected from the region to detect movement of the object and to generate tracking signals, and applies the tracking signals (i) to direct the active tracking system to move the tracking beam to follow the movement of the object, and (b) as input to the OCT scanning apparatus, to direct the OCT scanning apparatus to move the scanning beam to follow the movement of the object.